During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power both on an absolute basis and as a function of binder hardening, more rapid developability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, can be achieved by employing tabular grain emulsions. These advantages are demonstrated in Kofron et al U.S. Pat. No. 4,439,520.
An emulsion is generally understood to be a "tabular grain emulsion" when tabular grains account for at least 50 percent of total grain projected area. A grain is generally considered to be a tabular grain when the ratio of its equivalent circular diameter (ECD) to its thickness (t) is at least 2. The equivalent circular diameter of a grain is the diameter of a circle having an area equal to the projected area of the grain.
High chloride tabular grain emulsions are disclosed by Kofron et al. The term "high chloride" refers to grains that contain at least 50 mole percent chloride based on silver. In referring to grains of mixed halide content, the halides are named in order of increasing molar concentrations--e.g., silver iodochloride contains a higher molar concentration of chloride than iodide.
The overwhelming majority of tabular grain emulsions contain tabular grains that are irregular octahedral grains. Regular octahedral grains contain eight identical crystal faces, each lying in a different {111} crystallographic plane. Tabular irregular octahedra contain two or more parallel twin planes that separate two major grain faces lying in {111} crystallographic planes. The {111} major faces of the tabular grains exhibit a threefold symmetry, appearing triangular or hexagonal. It is generally accepted that the tabular shape of the grains is the result of the twin planes producing favored edge sites for silver halide deposition, with the result that the grains grow laterally while increasing little, if any, in thickness after parallel twin plane incorporation.
While tabular grain emulsions have been advantageously employed in a wide variety of photographic and radiographic applications, the requirement of parallel twin plane formation and {111} crystal faces pose limitations both in emulsion preparation and use. These disadvantages are most in evidence in considering high chloride tabular grains. It is generally recognized that silver chloride grains prefer to form regular cubic grains--that is, grains bounded by six identical {100} crystal faces. Tabular grains bounded by {111} faces in silver chloride emulsions often revert to nontabular forms unless morphologically stabilized.
Brust et al EPO 534,395, published Mar. 31, 1993, discloses radiation sensitive high chloride {100} tabular grain emulsions. As employed herein the term "high chloride {100} tabular grain emulsion" indicates a high chloride tabular grain emulsion in which the tabular grains accounting for at least 50 percent of total grain projected area have major faces lying in {100} crystallographic planes. The high chloride {100} tabular grain emulsions of Brust et al represent an advance in the art in that (1) by reason of their tabular shape, they achieve the known advantages of tabular grain emulsions over nontabular grain emulsions, (2) by reason of their high chloride content they achieve the known advantages of high chloride emulsions over those of other halide compositions (e.g., low blue native sensitivity, rapid development, and increased ecological compatibility--that is, rapid processing with more dilute developer solutions and rapid fixing with ecologically preferred sulfite ion fixers), and (3) by reason of their {100} crystal faces the tabular grains exhibit higher levels of grain shape stability, allowing the use of morphological stabilizers adsorbed to grain surfaces during emulsion preparation to be entirely eliminated. A further and surprising advantage of Brust et al is that the high chloride {100} tabular grain emulsion sensitivity levels can be higher than previously thought possible for high chloride emulsions.
Historically photographic applications requiring higher photographic speeds have been served by employing photographic elements containing silver iodobromide emulsions, since these emulsions can exhibit the most favorable speed-granularity relationships. With the improved speed-granularity relationships obtained using the high chloride {100} tabular grain emulsions of Brust et al, the realization has occurred that high chloride {100} tabular grain emulsions can be used for photographic applications, such as films for use in hand held cameras, that have traditionally been served by silver bromoiodide emulsions, allowing the advantages of the high chloride composition to be obtained in these applications. However, Brust et al, though improving the speed-granularity position of high chloride emulsions, still has not equalled the best speed-granularity relationships of silver iodobromide emulsions.